Hope you don't mind my chiming in. We haven't had hatchlings yet, but, if I were to choose a holdback I would first look for good solid bars in the body pattern. Even before color started showing up. Then, still early on, I would look for very robust specimens. Once you got a few guys with great bar patterns and great health, then watch for color.
So, initially I would hold back buys with great patterns, and true solid bars. Then, once the color started in, I would maybe sell the ones that looked a little ho-hum, and hold on to any with exceptionally vivid or unusual coloration. If there are several that meet that criteria, then perhaps choose the best one, or maybe the largest of the best.
Of course this all depends on how much room you have to hold on to several guys for a few extra months. I don't know how you would choose a holdback if you had to make the decision in the first 2 months- except by pattern, health, and bars.
Also, keep in mind that long term breeders may be looking for a specific coloration to add variation to the animals they already have in their breeding program.